1. Filed of the Invention
The present invention relates to a semiconductor light-emitting device using a nitride semiconductor or a like material, and more particularly, to a high reflective electrode structure concurrently satisfying a low contact resistance and a high reflectivity, and a flip-chip light-emitting device having the same.
2. Description of the Related Art
A nitride-based or GaN-based compound semiconductor, which is generally used for a visible light emitting device, is advanced to emit light in an ultraviolet light region for a white LED as well as visible light regions of blue and green. The nitride-based compound semiconductor is mainly classified into 1) a structure of extracting an upward light from the active layer, and 2) a structure of extracting a downward light passing through a transparent substrate such as a sapphire substrate.
In a flip-chip light emitting device having the structure of extracting light through the transparent substrate, reflectivity at an interface of a P-type electrode is of importance to reflect the upward light to direct downward.
In the meantime, it is advantageous that a light emitting device has a low operation voltage. At present, the most general method for lowering an operation voltage of the light emitting device involves decreasing resistance of a material layer formed between an electrode layer and an active layer. Especially, since a hole injection layer (that is, P-type semiconductor layer) and a P-type electrode are in Ohmic contact to each other in the flip-chip light emitting device, it is very desirable that the hole injection layer and P-type electrode have low Ohmic contact resistance formed therebetween so as to reduce the operation voltage.
FIG. 1 is a schematic sectional view illustrating a conventional flip-chip nitride semiconductor light-emitting device.
As illustrated in FIG. 1, the conventional flip-chip nitride semiconductor light emitting device 10 includes a sapphire substrate 11, an N-type GaN layer 12, an active layer 16 formed of InGaN, a P-type GaN layer 18, a nickel layer 20, a P-type reflective electrode 22 sequentially formed on the sapphire substrate 11, and an N-type electrode 14 formed on one side surface of the N-type GaN layer 12. The light emitting device 10 has a dual hetero-junction structure where the N-type GaN layer 12 functions as a cladding layer for a first conductive type, and the P-type GaN layer 18 functions as a cladding layer for a second conductive type.
Further, the nickel layer 20 is formed on the P-type GaN layer 18 to have a thickness of below about 10 nm, and functions as a contact metal layer for forming the Ohmic contact. Since the P-type reflective electrode 22 is formed of aluminum (Al) or silver (Ag), light transmitting the nickel layer 20 that is the contact metal is reflected at an interface between the P-type reflective electrode 22 and the nickel layer 20.
The conventional light emitting device may extract light without the nickel layer 20, using the P-type reflective layer 22 of material such as aluminum (Al) or silver (Ag) with a high reflectivity, and can achieve a high efficiency of light extraction. However, in case that the P-type reflective electrode 22 is directly formed on the P-type GaN layer 18 without the nickel layer 20 therebetween, the contact resistance is increased considerably. Accordingly, it is desirable that the contact metal layer 20 be formed on the P-type GaN layer 18 for forming the Ohmic contact, thereby reducing the contact resistance.
However, in the flip-chip nitride semiconductor light emitting device 10 having the nickel layer 20 as the contact metal, since light emitted from the active layer 16 formed passes through the nickel layer 20, and then is reflected at the interface between the nickel layer 20 and the P-type reflective electrode 22, and then again passes through the nickel layer 20 and the sapphire substrate 11 for extraction, a large amount of light is absorbed by the nickel layer 20. Therefore, the conventional flip-chip nitride semiconductor light emitting device 10 has a drawback in that it is very difficult to increase the reflectivity.
Since the nickel layer 20, which is the contact metal, is used to be in reliable contact with the P-type GaN layer 18, the thicker nickel layer 20 can provide a better contact with the P-type GaN layer 18. However, if the nickel layer 20 has a thickness of above 10 nm, it is difficult to have enough reflectivity.
Accordingly, the semiconductor light emitting device is required to have a reflection structure for maintaining the high reflectivity while maintaining the low contact resistance.